It is necessary to monitor the metering of oxygen in the blood during the mechanical respiration of patients, especially newborn and premature babies. A physiologically unadapted saturation, which is subject to great variations in terms of the degree of saturation, may lead to damage to the eyes with the negative consequence of partial or total blindness (retinopathy of premature: ROP) in case of an oxygen concentration of 98% to 100% in the blood lasting over a period of several minutes. Other side effects of a highly fluctuating oxygen concentration are permanent lung injury (ALI (acute lung injury)) as well as brain damage. The selection of the oxygen concentration combined with the other parameters of respiration such as respiration rate and minute volume are decisive for a physiologically correct respiration in adults as well; this means that the combination of the parameters set must be selected to be such that the carbon dioxide is removed from the patient's lungs by the gas exchange, such that the carbon dioxide is replaced by a correspondingly selected quantity of oxygen in the course of respiration. To ensure the respiration parameters, a blood gas analysis is performed in the clinical routine after connection to the respirator for the first rime and after operation with starting parameters. A blood sample is taken for this from the patient according to an invasive method and the oxygen and carbon dioxide concentrations are determined from the sample.
Continuous monitoring of the oxygen concentration (SaO2) by means of taking blood samples is not possible in clinical practice, and the oxygen saturation (SPO2) is available for a continuous monitoring. The measurement of the oxygen saturation (SPO2) is performed in a suitable manner by means of a pulse oximeter. The oxygen saturation is determined during such a measurement in a noninvasive manner according to the optical transillumination method on the extremities or on the ear lobe by means of suitable finger sensors or ear lobe sensors adapted to the site of measurement. A suitable pulse oximetric sensor is shown in US 2001029325A. The measurement is carried out by means of a mutual transillumination, for example, of the finger, using two wavelengths in the red and infrared ranges. Measuring arrangements designed in this manner are shown in US 2002177762 A, US 2008188733 A and U.S. Pat. No. 6,381,479 B. Besides the oxygen saturation, these pulse oximeters detect the heart rate as another measured variable. Typical sampling rates of such devices are in the range of 0.1 Hz to 2 Hz. To reliably maintain the oxygen saturation at a physiologically adapted level, the inspiratory oxygen fraction (FiO2) is very often adjusted manually during respiration on the respirator in clinical practice. Inclusion of values of an oxygen saturation in the blood to determine a suitable inspiratory oxygen fraction (FiO2) in the breathing air is described in the state of the art; for example, U.S. Pat. No. 4,889,116 describes an adjustment of FiO2 in a predetermined target range of the oxygen saturation. The arterial oxygen saturation SaO2 is used in U.S. Pat. No. 5,388,575 A as a calculated and estimated auxiliary variable in connection with a linear interpolation of the nonlinearity of the SPO2—SaO2 curve in order to move physiologically closer to the target range of FiO2 from the measured SPO2 value. The use of the past history of the oxygen saturation values by means of trend analysis is known from U.S. Pat. No. 5,388,575 A, U.S. Pat. No. 6,512,938 B2 and U.S. Pat. No. 6,761,165 B2. U.S. Pat. No. 5,365,922 describes a closed control loop with the use of an SPO2 sensor and a measuring device in the feedback of the control loop. Adjustment of the oxygen concentration leads to a response of the oxygen saturation only after a time lag in the closed control loop. Since the time lags depend on both the measuring arrangement, the type of gas metering and gas supply to the patient and the physiological and pathological constitution of the patient, designing the controller for a physiological and pathological patient constitution may lead to an unstable control characteristic, whereas the equivalent design of the controller for another physiological and pathological patient constitution leads to a response time of oxygen metering that is not acceptable from a physiological and therapeutic point of view or to an unacceptable permanent deviation of the oxygen saturation in the blood compared to a target range. U.S. Pat. No. 5,682,877 describes the inclusion of the oxygen saturation in the control loop of oxygen metering, in which time lags concerning the process of oxygen metering over time in the device between an SPO2 measuring site, arranged, for example, on the upper and lower extremities, and the lungs, as well as possible times, which are necessary for obtaining stable measuring conditions after a change in metering, and preset waiting times in the serial process of oxygen regulation and oxygen metering are also taken into account.
The assignment of a saturation value to a discrete state of the patient, e.g., hypoxia, and the use of this state as an input variable for the adjustment characteristic of the oxygen fraction FiO2, is likewise described in U.S. Pat. No. 6,512,938 B2. An improvement of oxygen metering, associated with a reduced fluctuation of the oxygen concentration in the blood, can be achieved with U.S. Pat. No. 6,512,938 B2 for a limited number of patients, whose clinical pictures are classified. For a closed control loop, using an SPO2 sensor, US 2008/0066752 A1 describes the taking into account of physiological time lags in the control loop in adjusting the oxygen metering in order to reduce the range of variation of the oxygen concentration.
It is necessary to take into account additional factors to further reduce the variation in the oxygen concentration.